As well safety valves which are controlled by a hydraulic control line extending to the well surface are set at greater depths, the hydrostatic head in the control line increases. This requires higher closing forces to overcome the hydrostatic head pressures at a given depth. This has been achieved in the past, as shown in U.S. Pat. No. 4,161,219 by providing a strong spring biasing force for closing the valve and providing a small area hydraulic piston and cylinder assembly. However, with high spring rates and relatively long travel to open the valve, the surface operating pressures can become excessive and are required to be extremely high in the event that the tubing pressure which also biases the valve to a closed position is close to the rated well tree pressure.
Also in the past, in an attempt to overcome the hydrostatic head problem in the control line, two lines were connected from the well surface to the valve. One of the lines is the control line to control the opening and closing pressure of the valve and the second line is to balance the hydrostatic head pressure. However, gas from the well migrates into the balance line past the various seals in the valve and lowers the density of the counterbalancing fluid in the balance line. In this event, the hydrostatic head in the control line, which is then unbalanced, opens the valve and the biasing closing forces cannot close the valve. Therefore, with gas in the balance line, the valve will fail in the open position which is undesirable. One feature of the present invention is to overcome the problem of gas migrating into the balance line.
While it is desirable to have a high closing pressure in order to insure that the safety valve will be able to close when needed, it is undesirable that the surface operating pressure in the control line be overly high. Another feature of the present invention is to provide a safety valve having a low opening pressure and a high closing pressure.